An animal's interactions with its external world during restricted periods of development have a profound and lasting influence on its behavioral capacities. Although the role of sensory experience in the development of learned behaviors is well established, the mechanisms whereby experience shapes neural circuits underlying such behaviors are not well understood. Songbirds, like humans, learn their species-typical vocalizations after hearing them during a specific period of development, and vocal behavior is controlled by highly localized neural circuits. We will use the songbird model system to investigate how endogenous and experiential factors regulate neural and behavioral development. (1) We will examine developmental changes in the expression of neurotrophins, a specific class of growth factors, in neural circuits that control vocal learning in order to generate hypotheses concerning their locus and mechanism of action. (2) We will test whether release of neurotrophins from axon terminals of active neurons can regulate the survival and/or differentiation of their post-synaptic target neurons. (3) We will investigate the electrophysiology of vocal-control circuits, focusing on developmental changes in regulation of excitatory synaptic transmission by neurotrophin growth factors in order to characterize patterns of functional communication between neurons. (4) We will study the morphology of axon terminals of individual neurons during vocal development to assess the specificity of connections among vocal-control neural circuits, and examine the influence of disruptions in vocal learning due to auditory deprivation on development of single axons. (5) We will assess the influence of neurotrophins on the development of individual axon terminals as a means of testing how highly ordered patterns of connectivity within vocal-control neural circuits and the specialized behaviors they subserve emerge during restricted sensitive periods early in development.